1. Field of the Invention
The present invention relates generally to fuel cells used for converting thermal energy into electrical energy. More particularly, the present invention relates to thermally regenerated fuel cells which are based upon the oxidation and reduction of hydrogen and utilize solvent-free electrolytes.
2. Description of Related Art
Thermally regenerated electrochemical systems have been investigated extensively since the late 1950's. In these systems, the working substances utilized in an electrochemical cell to generate electrical current are regenerated by providing thermal energy to the products of the electrochemical reactions. These systems are similar to secondary batteries in many respects except that regeneration of the electrochemically active reactants is accomplished thermally rather than electrically.
Thermally regenerated fuel cell systems which are based upon the oxidation and reduction of hydrogen are particularly useful because hydrogen electrodes are very efficient. Such electrodes are capable of supporting reasonably high current flow and they are well known in the industry. Representative fuel cells which utilize hydrogen electrodes are described in U.S. Pat. No. 4,810,596 issued to Ludwig in March, 1989 and assigned to the present assignee, ('596 Patent). Among the systems which are described in the '596 Patent is a prior fuel cell in which the cathode electrolyte is concentrated sulfuric acid and the anode electrolyte is diluted sulfuric acid. The difference in acid concentration between the two solutions is maintained by heating the concentrated solution to distill water generated at the cathode. A significant disadvantage to this fuel cell system is the inefficient distillation process, the bulky equipment required for the distillation, and the need to circulate large amounts of water. Additionally, aqueous based systems require an external hydrogen return from cathode to anode which is prone to leaks. A preferred method for transferring the hydrogen from the cathode to the anode is through a porous cell separator. However, the low surface tension in aqueous systems allows electrolyte to flood the pores of these porous separators.
Another system described in the '596 Patent is an improvement to the above-described fuel cell in which a buffered solution containing sodium sulfate and sodium bisulfate is substituted for the dilute acid. During operation, sodium bisulfate is generated at the anode and sodium sulfate is consumed. For regeneration, the sodium bisulfate is thermally converted to sodium sulfate, water, and sulfur trioxide. The sulfur trioxide is combined with water to regenerate sulfuric acid. This system is well suited for its intended purpose, however, the regeneration procedure requires the conversion of sodium bisulfate to sodium sulfate and sulfuric acid at a temperature of 450 degrees C. Consequently, this system is unsuitable for relatively low temperature conversion in thermally regenerated fuel cells.
A fuel cell which is thermally regenerated at a lower temperature is disclosed in U.S. Pat. No. 4,738,904 issued to Ludwig et al. in April 1988, assigned to the present assignee ('904 patent). It utilizes a fluid Bronsted acid and a fluid Bronsted base in the cathode and anode, respectively. The anion of the acid combines with the cation of the base to form a salt which is thermally regenerated at temperatures below 250.degree. C. This system, however, suffers from high electrolyte resistance and for most applications, water as an inert solvent is required.
Although the above-described fuel cells are well suited for their intended uses, there is a continuing need to provide thermally regenerated fuel cells which are not based upon aqueous electrolytes.
There is also a continuing need to provide thermally regenerated fuel cells which do not require the transfer of inert or aqueous solvents by energy consuming fractional distillation techniques. There is additionally a continuing need for high efficiency fuel cells which are capable of thermally regenerating the working electrolytes at relatively low temperatures of less than 200.degree. C. For example, a need exists in the automotive industry for a system which can produce electrical energy from the waste heat of an internal combustion engine.
There is also a need to provide a thermally regenerated fuel cell with low internal resistance and high electrolyte conductivity.